Correct wiring of a bilge pump may possibly save your boat from sinking, whether water is coming into the bilge from a leak or even from a heavy downpour. Follow these steps when wiring your bilge pump to ensure proper operation. When installing a bilge pump it is important to use the right gauge wire for the size of the pump---larger capacity pumps draw more current and need larger wires. The ABYC Wire Size Table will have this information once you know how many amps your pump draws and the length of the wires you will be using.
To wire a bilge pump with a float switch, run a wire from the negative battery terminal to the pump and run another from the other pump terminal to the float switch. From that same terminal on the pump run a wire to a three way switch. (One position is "On", one is "Off" and one is "Auto.") Run another wire from the float switch to the three way switch. From the three way switch run a wire to the positive terminal of the battery.
All wires should be supported every 18 inches with cable clamps or other means, and the ends of the supply wires (the ones connected to the battery) should have crimp-on ring terminals installed for proper connection to the battery terminals.
All wiring should be led up and out of the bilge area so that the least amount of wire will be exposed to water. An automatic bilge pump should be connected directly to the battery, since when the boat's power is turned off you still want the bilge pump to work.
To wire a bilge pump without the float switch, the circuit is the same, but the three way switch is replaced with a toggle switch, and there is only one wire connected to each terminal of the bilge pump--one going to the negative terminal of the battery and one going to the toggle switch, which has a wire led from it to the positive terminal of the battery (also with a fuse installed).
To get a good connection between the pump terminals and the wires leading to the battery, use crimp-on step-down butt connectors. These connections can be made watertight with a piece of heat shrink tubing, which you must slide over the wire before making the connection, and then position over the connection while you apply heat. Make sure the crimp-on ring terminals used in the connections to the battery will fit the battery posts.
Be sure to install a fuse on the positive side of the circuit, preferably very close to the battery.